Is blending power tube brands recommended?

Hello tube owners who are wiser than me!

I have a vintage Fisher 500c and have been running a matched quad of current production EH 7591s in it with fine results. However, I still have the original quad of Fisher branded 7591s that I believe actually came with the unit circa 1965! They still test strong.
So, on a lark I threw the Fishers back in and was quite surprised at how good they sounded! Warmer, much more "PRaT", and just more "liquid" especially in the mids. Soundstage was a bit shallower, they're a bit less detailed, and not as quiet, but I really enjoyed the change. EH tubes are still better in some respects though.
So could I run a pair of each safely and would I get the best of both sonically?
Thanks!
Lincoln